@cesar07 There is no right or wrong answer here. Whatever you decide is the right choice for you guys at this time. If you choose to terminate you can still conceive at a later time. A majority of abortions are from people who already have a child or children and are not in a place in their lives when they can have another. That doesn't mean they can't have another at a later stage.
Similarly, if you choose to keep it, things may not be as difficult as you're imagining financially. A lot of it depends on where you're living and what expenses you are expecting. Can you share some more info about this? What are the main short term expenses (lack of income, needing to get a second pram or car seat etc) and long term expenses (childcare, house size, 2 mouths to feed etc). Could you afford these things that you need? What are the chances you and/or your husband will be able to find work within the next 9 months? What happens if you don't? Do you have family around to help you look after the kids, cook some meals, etc?
I have a friend who has an 18 month old and a young baby. The main issue is that the oldest child is still at an age that he needs someone all the time. If you take your eyes off him for 2 minutes he'll be getting into all the drawers and cupboards. You need to watch him around the baby very carefully because he tries to hit and throw things. He isn't old enough to understand instructions or do anything for himself. He yells and bangs things when the baby is trying to nap. They don't have enough bedrooms in the house so everyone is sharing and waking each other up. It looks really really hard. BUT I also have another friend whose kids are a similar age gap but they're now 4 and 5 years old and it looks really awesome. The boys get along really well, do all the same things- for the mum it's basically just lugging 2 kids around to the same activities and things she'd be doing with #1 anyway.
The one thing I wouldn't worry too much about is your second point about somehow taking away from your first child. Whenever it is that you have a second, you'll be taking time away from your first child, but she'll also be gaining something. It doesn't matter whether she's 18 months, 3 years, 9 years etc. It will always be an adjustment. At 18 months she'll be very different to now at 9 months- much less like a baby, more like a toddler. Of course that comes with all its own challenges- she'll be mobile, clumsy, emotional, wilful, saying a few words. It's a fun age. But I personally would find it very hard dealing with a newborn and an 18 month old at the same time. But it also means you get it all out of the way quickly and they'll be close in age growing up. In some ways things would be easier down the track when you have 2 kids close in age- they will be interested in doing the same activities, they may play together very well and be the best of friends! They can even play with each others' friends.
At the same time, you could always always terminate this pregnancy and try again in a couple of years. Absolutely nothing to stop you doing that instead, and your child still gets all the benefits of a sibling without you struggling with the close age gap.
What does your heart say?